If you got the seat used it may have been built on a pan from a early 1100GS. In '95 the bikes had plastic tanks for a short while before BMW switched to metal tanks. The early seats are different to conform to the plastic tank. The gap you see would be due to the different shape of the plastic tank. I have had early seat pans on later metal tanked bikes and the gap is large like you describe.
Mike

Hmm. That is interesting. Thanks for the info, now at least if I want to sell it then I know how to market it correctly. Thanks Mike!